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TECHNICAL TRANSACTIONS 1/2017CZASOPISMO TECHNICZNE 1/2017

CHEMISTRYDOI: 10.4467/2353737XCT.17.003.6100

Grzegorz Kurowski (kurowski@chemia.pk.edu.pl)Otmar VogtJan Ogonowski

Institute of Organic Chemistry and Technology, Faculty of Chemical Engineering and Technology, Cracow University of Technology

Active ingredients in paint strippers

Substancje aktywne w preparatach do usuwania powłok lakierniczych

AbstractThe papers presents the study on the application of acetals as active ingredients in paint strippers. The formulation was prepared and the ability to wrinkling of the shell was rated. The ability to wrinkling of the shell was rated by examining the force required to detachment the coating.Keywords: paint stripper, active ingredients, acetals

StreszczenieW artykule przedstawiono badania dotyczące zastosowania acetali jako substancji aktywnych w preparatach do usuwania powłok lakierniczych. Sporządzono preparaty oraz oceniono ich zdolność do odspajania powło-ki. Zdolność do odspajania powłoki oceniano badając siłę potrzebną do oderwania powłoki.Słowa kluczowe: preparaty do usuwania powłok lakierniczych, substancje aktywne, acetale

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1. Introduction

Since the Council of the European Union has banned the use of methylene chloride in paint removers [1], it was necessary to find substances, which are able to replace this compound. Currently, active ingredients that are used include compounds, such as N-methylpyrrolidone [2–6] and benzyl alcohol [7–12] as well as alkylene carbonates, such as propylene carbonate or ethylene carbonate [13–15]. Furthermore, carboxylic acid esters are also used. In patents, benzoic acid methyl ester [16] as well as methyl esters of dicarboxylic acids, such as succinic acid, glutaric acid and adipic acid [17–19], are described. Moreover we can use such esters like: ethyl acetate, butyl acetate, ethyl 3-ethoxypropionate and 1-methoxy-2-propyl acetate. However, the most preferred is ethyl 3-ethoxypropionate [20, 21]. The patents also describe such compounds as alkyl derivatives of carboxylic acid amides, dicarboxylic acid and hydroxycarboxylic acids [22], alkoxylated aromatic alcohols [23, 34].

Research on this topic is also being conducted at the Institute of Organic Chemistry and Technology of the Department of Chemical Engineering and Technology at the Cracow University of Technology. In our research, we study the usefulness of acetals as active ingredients.

The topic of this study was to check the influence of different active substances from the group of acetals on the ability to remove the coating. For comparison, we used the most commonly used active ingredients (N-methylpyrrolidone and benzyl alcohol).

2. Materials and methods

2.1. The paint removers

In the study, paint removers based on the author’s recipe were used. The way of their preparation was different for the used active ingredient. For the study, we used substances with a different partition coefficient (logP). Due to the used different active ingredients, we used preparations in two forms - gel and emulsion..

In the study, the following substances were used: N-methylpyrrolidone (logP –0.40), dimethoxymethane (logP –0.26), dioxolane (logP –0.06), benzyl alcohol (logP 1.03), diethoxyethane (logP 1.14), cyclohexanone ethylene ketal (logP 1.35), benzaldehyde ethylene acetal (logP 1.63), dipropoxyethane (logP 2.21), dibutoxyethane (logP 3.27).

2.2. Preparations in gel form

Dioxolane and N-methylpyrrolidone were used in the gel preparation as active ingredients.

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Table 1. The composition of the gel preparation

Substance Composition [% wt.]

Active ingredient 42.2

Adduct 14.0

Water 14.0

Frakol 13.5

Ester solvent 6.7

Modisurf Clarity 4.8

Methocel 4.8

2.3. Preparations in emulsion form

Table 2. The composition of the emulsion preparationSubstance Composition [% wt.]

Active ingredient 45.6

Methyl esters of rapeseed oil 13.6

Hydrogen peroxide 13.6

Candelila wax 9.1

Ester solvent 6.8

Frakol 6.8

Tween 40 2.9

Glycerol monostearate 1.6

Dimethoxymethane, dioxolane, benzyl alcohol, diethoxyethane, cyclohexanone ethylene ketal, benzaldehyde ethylene acetal, dipropoxyethane, dibutoxyethane were used in the emulsion preparation as active ingredients.

2.4. Plates

In the study, we used five types of plates. Two plates (blue, white) were delivered by Fiat Auto Poland Company in Tychy. The blue plate was painted with solvent-based acrylic paint. The white plate was painted with waterborne acrylic paint. The other two plates (green and brown) were painted with melamine formaldehyde paints. The last plate (epoxy) was painted with epoxy paint. The three last paints are used for roof covering.

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2.5. The study of coating detachment

The paint remover was applied on the plate. In each case, the formulation was applied to the surface of 9 ± 0.3 cm2. Due to the next steps of the procedure, we adopted a uniform square shape for the area of application of the preparation. The preparation was left on the surface for 24 hours. After this time, the preparation was removed and the effect on the surface was rated. A metal badge with an area of 9 ± 0.3 cm2 was glued next to the coating. The glue was left for 48 hours to dry completely. After this time, the force required to detach the coating was measured. The measurement was performed on Zwick 1445 apparatus intended for testing the strength of materials. In the study, a constant crosshead speed of 50 mm/min was applied.

3. Results and discussion

3.1. Results of study of the interaction on the coating

In the case of the green plate, all active ingredients caused wrinkling of the shell. A similar situation took place in the case of the brown plate. An exception was diethoxyethane, which did not cause wrinkling of the shell. A different situation occurred in the case of the blue, white and epoxy plates. In those cases, wrinkling of the shell caused only dioxolane (emulsion). The wrinkling of the shell also caused dimethoxymethane, but only in the case of the blue plate. In the case of the epoxy plate, wrinkling of the shell also caused the preparation with benzyl alcohol. In other cases, the preparation did not cause any wrinkling of the shell.

3.2. Results of coating detachment study

In the case of the blue plate, we can see a reduction of the force needed to detach the coating that treated the preparation compared to the coating that did not treat the preparation. The average force required to pull-of the coating, which did not treat the preparation, is 216 N. However, not all layers of the coating are detached. The detachment of all layers of the coating was possible after the action on the coating formulations containing dioxolane (gel), diethoxyethane and benzaldehyde ethylene acetal. The force required to pull-of the coating is 158 N  for dioxolane, 182 N  for diethoxyethane and 136 N  for benzaldehyde ethylene acetal. The preparations with cyclohexanone ethylene ketal and dibutoxyethane caused softening of the shell. It makes it possible to detach the coating of the silver layer with an average force of 148 N for cyclohexanone ethylene ketal and 121 N for dibutoxyethane.

Comparing the results with the properties of the compound (partition coefficient of o/w logP), we can see that, with an increase of the partition coefficient, the susceptibility to wrinkling of the shell is lower. The preparations containing dioxolane (logP –0.06) and

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dimethoxymethane (logP –0.26) caused wrinkling of the shell. The preparations containing diethoxyethane (logP 1.14) and benzaldehyde ethylene acetal (logP 1.63) caused softening of the shell. This makes it possible to detach all layers of the coating. The preparations containing dipropoxyethane (logP 2.21) and dibutoxyethane (logP 3.27) also caused softening of the shell, but this makes possible to detach only one layer of the coating.

Blue plate

Table 3. Results of coating detachment study for the blue plate

Active ingredient Force

CommentskPa N

– 246.1 216.0 A part of the coating was detached

Dimethoxymethane The coating was wrinkled

Dioxolane (gel) 176.2 158.6 All layers of the coating were detached

Dioxolane (emulsion) The coating was wrinkled

Diethoxyethane 209.5 182.2 All layers of the coating were detached

Cyclohexanone ethylene ketal 159.0 147.9 All layers of the coating were partially detached

Benzaldehyde ethylene acetal 146.6 136.4 All layers of the coating were detached

Dipropoxyethane 147.9 133.1 Coating intact

Dibutoxyethane 139.5 121.4 Coating was detached to the silver layer

Fig. 1. The plate after study of coating detachment; 1 – reference sample, plate treated the preparation with 2 – dioxolane (gel), 3 – diethoxyethane, 4 – cyclohexanone ethylene ketal, 5 – benzaldehyde ethylene acetal, 6 – dipropoxyethane, 7 – dibutoxyethane

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White plate

Table 4. Results of coating detachment study for the white plate

Active ingredient Force

CommentskPa N

– 269.0 242.1 Coating intact

N-methylpyrrolidone 226.4 197.0 The first layer of the coating was detached

Dimethoxymethane 162.3 151.0 All layers of the coating were detached

Dioxolane (gel) 306.3 284.9 Coating intact

Dioxolane (emulsion) The coating was wrinkled

Benzyl alcohol 152.9 142.2 All layers of the coating were detached

Diethoxyethane 63.0 56.6 All layers of the coating were detached

Cyclohexanone ethylene ketal 243.5 226.4 All layers of the coating were detached

Benzaldehyde ethylene acetal 358.2 333.1 All layers of the coating were partially detached

Dipropoxyethane 155.7 140.2 Coating intact

Dibutoxyethane 209.8 195.2 Coating intact

Fig. 2. The plate after study of coating detachment; 1 – reference sample, plate treated the preparation with 2 – N-methylpyrrolidone, 3 – dimethoxymethane 4 – dioxolane (gel), 5 – benzyl alcohol 6  – diethoxyethane, 7  – cyclohexanone ethylene ketal, 8  – benzaldehyde ethylene acetal, 9 – ipropoxyethane, 10 – dibutoxyethane

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In the case of the white plate only, dioxolane (emulsion) caused wrinkling of the shell. In four causes, the activity caused softening of the shell, making the detachment of the coating possible. We can see that, with increasing logP, the force, which is needed to detach the coating, increases from 151 N  for dimethoxymethane (logP –0.26) to 333 N for benzaldehyde ethylene acetal (logP 1.63). The exception is diethoxyethane. For this compound, the force needed to detach the coating is 56.6 N. A further increase of logP makes the preparation not soften the coating. In this case, the adhesion forces of the paint to the base are stronger than the adhesion forces of the glue to the coating. Therefore, we cannot detach the whole shell, but only small fragments. Such a situation took place in the case of the reference sample and the sample with dipropoxyethane and dibutoxyethane.

Green plate

In the case of the green plate, all substances caused a significant reduction of the forces required to detach the coating. Only the preparation with N-methylpyrrolidone did not cause a  reduction of the forces required to detach the coating. However, it softened the coating, making it possible to remove all of the top layer.

Fig. 3. The plate after study of coating detachment; 1 – reference sample, plate treated the preparation with 2 – N-methylpyrrolidone, 3 – dimethoxymethane 4 – dioxolane (gel), 5 – dioxolane (emulsion), 6  – benzyl alcohol 7  – diethoxyethane, 8  – cyclohexanone ethylene ketal, 9  –  benzaldehyde ethylene acetal, 10 – dipropoxyethane, 11 – dibutoxyethane

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In this case, regardless of the partition coefficient, the substances caused wrinkling of the shell. The only exception is N-methylpyrrolidone. In its case, despite a low value of partition coefficient, it did not cause wrinkling of the shell.

Table 5. Results of coating detachment study for the green plate

Active ingredient Force

CommentskPa N

– 513.7 446.8 A part of the coating was detachedN-methylpyrrolidone 567.4 510.7 The first layer of the coating was detachedDimethoxymethane 37.3 33.5 The coating was wrinkledDioxolane (gel) 139.1 121.0 The coating was wrinkledDioxolane (emulsion) 66.0 60.2 The coating was wrinkledBenzyl alcohol 89.1 77.5 The coating was wrinkledDiethoxyethane 120.8 108.7 The coating was wrinkledCyclohexanone ethylene ketal 32.2 30.0 The coating was wrinkledBenzaldehyde ethylene acetal 66.5 56.4 The coating was wrinkledDipropoxyethane 60.0 54.0 The coating was wrinkledDibutoxyethane 58.8 54.7 The coating was wrinkled

Brown plate

Table 6. Results of coating detachment study for the brown plate

Active ingredient Force

CommentskPa N

- 507.4 456.6 A part of the coating was detachedN-methylpyrrolidone 754.5 679.0 The first layer of the coating was detachedDimethoxymethane - - The coating was wrinkledDioxolane (gel) 392.9 353.6 The coating was wrinkledDioxolane (emulsion) 169.0 152.1 The coating was wrinkledBenzyl alcohol 8.3 7.4 The coating was wrinkledDiethoxyethane 305.8 275.2 The coating was wrinkledCyclohexanone ethylene ketal 83.4 77.5 The coating was wrinkledBenzaldehyde ethylene acetal 37.4 34.8 The coating was wrinkledDipropoxyethane 268.3 241.4 The coating was wrinkledDibutoxyethane 173.1 155.8 The coating was wrinkled

The same situation as in the green plate occurred in the case of the brown plate. We can see a decreasing force required to detach the coating, from 465 N for the reference sample, to 7.4 N  for benzyl alcohol. The exception is a  test in which the coating was treated with the preparation containing N-methylpyrrolidone. In this case, the coating was not wrinkled. However, it softened the coating, making it possible to remove all of the top layer.

Similar in this case, regardless of the partition coefficient, the substances caused wrinkling of the shell. As a  result, the force required to detach the coating was reduced. The only exception is N-methylpyrrolidone. In its case, despite a low value of partition coefficient, it did not cause wrinkling of the shell.

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Epoxy plate

Table 7. Results of coating detachment study for the epoxy plate

Active ingredient Force

CommentskPa N

– 650.7 605.1 A part of the coating was detached

N-methylpyrrolidone 194.6 181.0 A small part of the coating was detached

Dimethoxymethane 63.0 52.9 A part of the coating was detached

Dioxolane (gel) 249.6 224.6 A part of the coating was detached

Dioxolane (emulsion) The coating was wrinkled

Benzyl alcohol The coating was wrinkled

Diethoxyethane 599.9 521.9 A part of the coating was detached

Cyclohexanone ethylene ketal 249.3 224.2 A part of the coating was detached

Benzaldehyde ethylene acetal 506.2 439.4 A part of the coating was detached

Dipropoxyethane 411.0 369.9 A part of the coating was detached

Dibutoxyethane 204.9 190.6 A part of the coating was detached

Fig. 4. The plate after study of coating detachment; 1 – reference sample, plate treated the preparation with 2 – N-methylpyrrolidone, 3 – dimethoxymethane 4 – dioxolane (gel), 5 – dioxolane (emulsion), 6  – benzyl alcohol 7  – diethoxyethane, 8  – cyclohexanone ethylene ketal, 9  –  benzaldehyde ethylene acetal, 10 – dipropoxyethane, 11 – dibutoxyethane

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In case of epoxy plate only dioxolane and benzyl alcohol caused wrinkling of the shell. In other cause substances caused softening of the shell. Thereby the force needed to detachment of the coating was less than in the case of coating didn’t treat the preparation. In this case, we also can see that with increasing logP, the ability to wrinkling of the shell is decreased. However, this is not as pronounced as in the case of acrylic coatings.

4. Conclusions

All the used active substances caused softening or wrinkling of the shell. The best results were obtained on plates painted with melamine-formaldehyde paint. In the case of the green and brown plates, most of the test substances caused wrinkling of the shell. Thereby, the force needed to detach the coating was significantly lower than in the case of the reference sample. The exception is N-methylpyrrolidone, which did not cause wrinkling of the shell in any of the cases. In the case of the plate with acrylic (blue and white) and epoxy coating, only dioxolane caused wrinkling of the shell. In the case of the blue plate, wrinkling of the shell was also caused by dimethoxymethane. In other cases, we observed softening of the coating. Additionally, we can see that, with an increase of the partition coefficient, the ability of wrinkling the shell is lower. The study shows that dioxolane and dimethoxymethane are the best substances to replace methylene chloride.

Fig. 5. The plate after study of coating detachment; 1 – reference sample, plate treated the preparation with 2 – N-methylpyrrolidone, 3 – dimethoxymethane 4 - dioxolane (gel), 5 – dioxolane (emulsion), 6  – benzyl alcohol 7  – diethoxyethane, 8  – cyclohexanone ethylene ketal, 9  – benzaldehyde ethylene acetal, 10 – dipropoxyethane, 11 – dibutoxyethane

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